with astrosat-czti
TRANSCRIPT
Prospects of Hard X-ray polarimetry
with Astrosat-CZTI Santosh Vadawale
Physical Research Laboratory, Ahmedabad
“X-ray Polarisation in Astrophysics – A new window about to open”, 28 August 2014, Stockholm, Sweden
On behalf of CZTI team:
A. R. Rao (TIFR), D. Bhattacharya, V. Bhalerao (IUCAA), S. Sreekumar (VSSC)
Astrosat
Introduction
CZT-Imager
X-ray Polarimetry with CZTI
Proof-of-Concept
Geant4 simulations
Experimental verification
Expected sensitivity
Summary
for Multi-wavelength Astronomy
Collaborative project of
Many premier Indian research institutes
+ two international parterres
India’s first dedicated satellite
650 km, near equatorial orbit Launch by Indian PSLV in 2015 (Q2/Q3) Observatory class, Proposal driven mission from 2nd year
Simultaneous multi-wavelength monitoring of broad range of cosmic sources.
Sky surveys in the hard X-ray and UV bands.
Broadband spectroscopic studies of X-ray binaries, AGN, SNRs, clusters of galaxies and stellar coronae.
Studies of periodic and non-periodic variability of X-ray sources.
Monitoring the X-ray sky for new transients.
Main scientific focus
LAXPC: Large Area X-ray Proportional Counters Aeff ≈ 6000 cm2; FOV =1° X 1°; 3-80 keV; Res: 9% @ 22 keV
CZTI: Cadmium-Zinc-Telluride Imager with Coded Aperture Mask Aeff ≈ 500 cm2; FOV = 6° X 6°;10 – 100 keV; Res: 6% @ 60 keV
SXT : Soft X-ray Telescope using conical-foil mirrors Aeff ≈ 200 cm2 ; FOV = 0.5°; (~3‘ res); 0.3-8 keV; Res: 2% @ 6 keV
SSM : Scanning Sky Monitor with 3 PSPCs and CAM Aeff ≈ 30 cm2 (each); 2-20 keV. Res: 20 % @ 6 keV
UVIT : Ultraviolet Imaging Telescope Two telescopes each with 38 cm aperture; N-UV, F-UV and visible
bands.
Five Instruments
Coded Mask imaging with pixilated CZT detectors
Total area 1024 cm2
Total pixels 16384
Pixel size 2.5 x 2.5 mm2
Field of view 6° x 6°
Angular resolution 8'
Energy resolution 6 % (@60 keV)
Sensitivity 3σ detection of 0.5 mCrab source in 104 s
4 x 4 cm2 (16 x 16 pixel)
5 mm thick
ASIC based readout (2 x 128 ch. ASIC)
Total 64 modules (16 in 4 quadrants)
Orbotech CZT modules
Pixilated detectors in principle can measure polarization Many attempts to measure polarization from instruments not
optimized for polarimetry (RHESSI, Integral-IBIS, Integral-SPI)
Can Astrosat CZTI measure X-ray polarization? May be above 100 keV (due to scattering probability / threshold)
(Capability in modules? Preserved in data? Meaningful sensitivity? )
For scattering polarimetry with Pixilated detectors azimuthal bins are unequal
Electric field vector of the incident photon
Low Z scatterer
Surrounding detector
Probability
Verify multi-pixel capability of CZTI modules
~70 % Polarized beam produced by 90° Compton scattering of 122 keV X-rays from Co57
Energy of polarized X-rays ~100 keV
Time tagging 20 µs
Time between two events Adjacent double pixel events, E2/E1>2
Co57 Source Scatterer
CZT
Polarized X-rays
CZTI Modules can detect simultaneous multi-pixel events
Simulate interaction of photons with 40x40x5 mm3 CZT (Low Energy Electro-magnetic processes)
Energy range 100 to 300 keV in 10 steps Unpolarized + 100 % polarized beam at 9 angles 10 Million photons at each step Record absolute positions of the primary
interaction and up to 5 secondary interactions Pixilate with 2.5 mm pixel Investigate number of pixels as well as number / type of interactions
Modulation curve has only 8 bins, which have non uniform angular coverage
Can be normalized with modulation curve for unpolarized beam
100 % polarized
Unpolarized
(Lei et al., 1997)
Generate 8 bin azimuthal histogram with low energy pixel as scattering pixel
Pol.Angle Average MF (%)
0 36.83
11.25 40.52
22.50 48.46
33.75 55.19
45.00 58.05
56.25 55.33
67.50 48.47
78.75 40.34
90.00 36.82
Modulation factor is now dependent on the polarization angle Figure of Merit is function of both energy and polarization angle
Significant modulation Polarization measurement possible Angle dependent modulation opportunity of cross verification
Detection of polarized X-rays with CZT-I Flight electronics
Polarized X-rays by Compton scattering of 356 keV from 133Ba in 70 – 107 deg. Incident energy: ~190 – 240 keV ~70% polarized beam
Measurements at four different incident angles 0°, 45°, 90° and 135° + unpolarized beam (direct 356 keV)
Modulation curve for unpolarized beam
0 degree 45 degree
90 degree 135 degree
0 degree 45 degree
90 degree 135 degree
Observed and simulated modulation factors match within 1% after addition of the instrumental background
Simulations with 100 % polarized beam do indicate realistic modulation factors
Angle Obs. Sim.
0° 19.68 20.74
45° 36.99 37.43
90° 11.96 22.06
135° 36.83 38.40
Modulation factors (%)
Observed modulation factor is less then simulated one for 100% polarized X-rays As expected because of partially polarized beam
Verify with Geant4 simulation of the experimental setup
All double pixel events satisfying filtering criteria Inst + Inst, Src + Src, Bkg + Bkg and their combinations Compton scattering of cosmic X-ray background photons
CZTI mask and shielding designed up to 150 keV
Most important component (99.99 %) is Compton scattering of cosmic diffuse X-ray background Different for different module due to varying shielding Estimated for each module separately
Innermost module Outermost module Two sides open, two sides shielded
Few simplifying assumptions about support structure
Effective polarimetric background is equivalent to ~1 Crab
Systematics effects are not important
Stable and easily measurable background
Different sensitivity at different polarization angle Any measurement can be verified at different S/C role angle
Slightly different sensitivity for sub-arrays Sub-array selection can be optimized at analysis stage
Too demanding on source intensity and exposure time?
Expected MDP
1 Crab, 1 MS
Known PA ~2%
Unknown PA ~8%
0.5 Crab, 500 ks
Known PA ~7%
Unknown PA ~20%
(not so bad for highly polarized sources)
Uncertainty in Pol. Deg.
depend on PA
determined by fitting to simulated library
For bright and highly polarized sources
polarization can be detected 50 – 100 ks
Scientifically meaningful?
Pulse phase resolved polarimetry Crab
Jet contribution to X-rays in BHB Cyg X-1, GRS1915+105, Transients
GRB polarimetry after studying the off-axis response (under progress)
100 – 250 keV polarimetry with Astrosat-CZTI is possible for bright sources MDP of ~8 % for a Crab like source in 1 Ms For highly polarized source, polarization signature can
be detected 50 – 100 ks
No additional requirement, only from available raw CZTI data in the standard mode
Polarimetric energy range (100 – 250 keV) is not the primary spectroscopic energy range Mask and other structural elements are transparent
Polarimetry is also possible for off-axis sources!! Good opportunity for GRB polarimetry
Real test Actual background level, No polarization
Thanks…
Can Astrosat CZTI measure X-ray polarization? Multi-pixel capability in the Orbotech CZT modules?
If yes, whether CZTI will have meaningful sensitivity?
Polarimetric information preserved by data processing?
Significant Compton scattering probability ~ 10 – 20 % in 100 – 300 keV
Compton polarimetry can be applicable for CZTI at energies (> 100 keV) beyond its primary spectroscopic energy range